Method and apparatus for atomising liquid media

ABSTRACT

There is disclosed apparatus for atomizing liquid media comprising an ultrasonic gas atomization nozzle ( 11 ) having a gas flow path ( 12 ) from a plenum chamber ( 17 ) which flow path is straight and is provided with a plurality of resonance cavities ( 31 ).

This invention relates to methods and apparatus for atomising liquidmedia, and also to making polymer powder.

Conventionally, polymer powder is made by grinding extruded polymerpellets, often under cryogenic conditions. Powder size distribution andpowder morphology are difficult to control, while the process isexpensive and energy-intensive. Moreover, the grinding equipment cancontaminate the product, which is also susceptible to environmentalpollution.

Methods and apparatus for atomising liquid media are known for examplefrom U.S. Pat. No. 5,228,620 and earlier publications, and are used e.g.to produce metal powder by atomising a molten metal stream into dropletswhich solidify into spherical or nearly spherical particles. The mostimportant characteristics of atomised powders are their morphologyshape, size and size distribution. The powder size and morphologysubsequently influences the engineering properties, i.e. flowability,packability, compressibility, etc., and the size distribution indicatesthe yield of useful material available for a specific application. It istherefore desirable to control the average particle size, themorphology, and the powder size distributions produced duringatomisation.

Prior to the invention, efforts in this area have resulted in thedevelopment of several techniques. One method used a standing ultrasonicwave generated between two ultrasonic transmitters to disintegrate amolten material into fine droplets (European Patent No. 0 308 600). Theother design is an ultrasonic gas atomisation device, in which the gaschannel incorporates a resonance cavity (Hartman shock tube) in order tocreate a high frequency pulse in the gas. The atomiser makes use of thecombination of high frequency pulsed gas pressure and supersonic gasstreams will promote efficient atomisation of the molten material,resulting in a narrow spread of fine droplet size (U.S. Pat. No.2,997,245). However, the amount of gas delivered by an atomisationnozzle is clearly one of the most important design parameters. Theinitial design has suffered from a major disadvantage, in that itrequires high operating gas pressure (from 6.5 MPa to 12 MPa) (U.S. Pat.No. 5,228,620). Abrupt frictional losses at areas in the channel arefound to be 36% in total pressure between the plenum chamber and nozzleexit in the Unal technical article (#1) “Frictional Losses in UltrasonicGas Atomisation Nozzles”, Powder Metallurgy, Vol. 33, No 3, pp.327-333(1990).

The present invention provides methods and apparatus for atomisingliquid media that overcome at least some of the problems of the priorart.

The invention comprises, in one aspect, apparatus for atomising liquidmedia comprising an ultrasonic gas atomisation nozzle having a gas flowpath from a plenum chamber which flow path is straight and is providedwith a plurality of resonance cavities.

The resonance cavities may be spaced apart along the gas flow path, andmay be inclined to the gas flow path in the sense of being convergedtherewith in the direction of gas flow.

The resonance cavities may be such as will impose an ultrasonicfrequency on the gas flow, which may be in the range 20-60 KHz.

The gas flow path may comprise an annular convergent nozzle and theresonance cavities then comprise cylindrical cavities formed in bothinterior walls of the annular nozzle. The diameters of the cavities maybe between 1/12 and ⅛ of the mean nozzle diameter. There may be betweenten and sixty cavities in such an arrangement spaced along and aroundthe annular nozzle.

The gas flow path may, however, comprise a multi-jet arrangement, andthe diameters of the cavities may be between 1/10 and ⅓ of the diameterof the jet passages into which they open. There may be between two andeight cavities in each jet. There may be between four and twenty jetsspread on one circle of radius around a liquid stream. The jets may bearranged in more than one angle toward liquid stream to performmulti-stage atomisation.

The cavities may be oriented at between 10° and 60° to the flowdirection through the nozzles.

The invention also comprises a method for atomising a liquid mediumcomprising impinging a flow of the liquid medium with a high-velocitygas stream with a superimposed ultrasonic frequency generated byresonance in the gas stream.

The ultrasonic frequency may be in the range 20-60 KHz, and thehigh-velocity gas flow may be at supersonic velocity.

The nozzle used may be of any type including free-fall and confinedtypes, annular and multi-jet nozzles, and may be of any miniature type,including inhalers and spray can nozzles.

Such nozzles may also be used, with the invention, to atomise variousliquids including molten metals, polymer melts, solvent based solutions,and other forms of liquids. A liquid may be formed by melting in acrucible or an extruder or dissolving in a solution, and may bedelivered to a die to form liquid streams.

In particular the invention also comprises a method for producingpolymer powder comprising melt extruding a polymer material andimpinging a high velocity gas stream on to the molten extrudate.

A single liquid stream may be impinged while in free fall from a die. Aliquid stream may comprise a film or filaments, in which latter case thefilaments may emerge as sheet or ribbon from a line of spinnerettes. Thefilm or sheet- or ribbon-like liquid stream, may be impinged on bothfaces by gas streams.

The gas stream velocity may be up to Mach 2.

The die may comprise heater arrangements to ensure the liquid is evenlyheated, and still molten in the region of impingement.

Air, nitrogen and argon may be used as atomising gas. Atomising gas maybe heated by a gas heater to atomise certain types of liquids. Using theinvention, the cost of special gases—such as nitrogen and argon—used ingas atomisation, can be substantially reduced. Not only is less gasused, but the maximum working pressure, of about 17 bar, generated fromconventional cryogenic supply of such gases is suitable for use withmethods and apparatus of the invention, avoiding the need for highpressure cryogenic pumping and high pressure storage vessels used inconventional gas ultrasonic atomisation. The gas used should, of course,not adversely react on or with the atomised polymer or other material.

The invention also comprises apparatus for making polymer powdercomprising a die from which polymer is extruded and nozzle meansimpinging a high velocity gas stream on the extrudate from the die.

The die may comprise a slit for extruding a film or a line ofspinnerettes for extruding a sheet or ribbon of filaments.

The nozzle means may comprise a slit-form nozzle either side of the diedirected towards the issuing extrudate. The nozzle mans may impinge thegas stream at an angle to the issuing extrudate so as to have acomponent of velocity in the direction of flow of the extrudate. Thenozzle means may form a V-shaped gas stream with an included anglebetween 30° and 90°.

The die may comprise heater arrangement to ensure the extrudate isevenly heated and still molten in the region of impingement.

The invention also includes powder, inter alia polymer powder, made bymethods or apparatus as herein disclosed. Such powders may becharacterised by comprising spherical or nearly spherical particles.

Methods and apparatus for atomising liquid media according to theinvention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a diagrammatic cross-section of a conventional flow channel;

FIG. 2 is a diagrammatic cross-section of a flow channel modified inaccordance with the present invention;

FIG. 3 is a detail not shown on the cross-section of FIG. 2;

FIG. 4 is a cross-section like FIG. 2 of another type of gas flowarrangement;

FIG. 5 is a comparative graphical depiction of particle sizedistribution of a typical product of a prior art process and a processaccording to the invention;

FIG. 6 is a cross-section of a melt die with gas stream nozzle means;

FIG. 7 is a view on arrow A of FIG. 6 of a first embodiment;

FIG. 8 is a view like FIG. 7 of a second embodiment; and

FIG. 9 is a graphical depiction of particle size distribution and atypical polymer product of a process according to the invention.

FIG. 1 illustrates a conventional gas atomisation nozzle 11, followingU.S. Pat. No. 2,997,245. The flow channel 12 comprises first and secondlegs 14, 15, joined at right angles, with a resonance cavity 16. Theabrupt change in the direction of flow between the two legs 14, 15 givesrise to considerable energy loss and limits nozzle efficiency.

FIG. 2 shows an improved design according to the invention in which theflow channel 12 has a single straight line leg from the plenum chamber17 to the nozzle exit 18. Elimination of the right-angled legarrangement of FIG. 1 improves the efficiency of the arrangement byeliminating energy losses involved in redirecting the direction of gasflow.

Not shown in FIG. 2 are alternative arrangements for generatingultrasonic frequency sound in the gas flow. These are indicated,however, in FIG. 3, where more resonance cavities 31 are shown openinginto the flow channel 12.

FIG. 3 shows opposed cavities 31 in a circular section jet flow channel12, the cavities 31 comprising cylindrical bores having a diameter ‘d’of 1/10 to ⅓ of the diameter ‘D’ of the channel 12. The cavities 31could be of other shapes, but it is easier to machine circular-sectioncavities usually.

In a convergent annular type nozzle, the cavities 31 would be much asillustrated in FIG. 3 but spaced apart circumferentially around theannular nozzle as well as lengthwise along the flow path.

For annular nozzles the bore diameter of the cavities can be between1/12 and ⅛ of the mean nozzle diameter.

Between two and eight resonance cavities can usually be arranged in eachjet of a multi-jet arrangement; between ten and sixty resonance cavitiescan be used in annular nozzle arrangements.

The geometry, distribution and number of resonance cavities willdetermine the intensity and frequency of the ultrasonic superimposition.Typical frequencies are 20-60 KHz, produced in a nitrogen gas streamgenerated by a plenum pressure between 1.4 and 1.7 MPa at up to Mach 2.

FIG. 4 illustrates a confined type nozzle (which may be either annularor multi-jet) according to U.S. Pat. No. 3,252,783 and U.S. Pat. No.5,228,620 adapted to the present invention.

In a typical arrangement a melting furnace was charged with 30 Kg of 316stainless steel, melted by induction and heated to a temperature of1600° C. Eight gas jet discharge orifices of free fall type werearranged to define an apex angle of 45°. The nozzles were supplied withnitrogen gas at 1.4 MPa. For comparison, nozzles with and withoutresonance cavities were used. In nozzles with cavities, there were six,each of 1 mm diameter uniformly arranged in each gas channel, formed atan angle of 15° to the direction of the channel.

Atomised droplets were collected after solidifying and size classified,the results being shown in FIG. 5. About 40% by weight of the particlesproduced by the nozzles with resonant cavities according to theinvention were of less than 38 μm diameter, compared to only about 15%of those produced by nozzles without resonant cavities, indicating thatthe ultrasonic superimposition produced by the resonant cavities hassignificantly enhanced the atomisation efficiency of the nozzles.

The FIGS. 6 to 8 illustrate apparatus for atomising liquid streams e.g.of polymer material comprising a die 111 from which a melt 112 isdelivered in the form of a film (FIG. 7) or a sheet or ribbon offilaments (FIG. 8), and gas stream nozzle means 113 impinging a highvelocity, e.g. Mach 1 or above, stream of gas on either side of the melt112.

The die 111 has a heater arrangement shown diagrammatically as anelectric resistance element 114 to ensure the melt 112 is evenly heatedand molten where the nozzle arrangement 113 impinges the melt 112.

The nozzle arrangement 113 comprises nozzles 113 a directed at the melt112 from either side thereof and angled so that the gas stream from eachhas a component velocity in the direction of flow of the melt 112, whichis itself in free fall from the die 111. The nozzles 113 a are outletsfrom plenum chamber means 113 b and are directed so as to form aV-shaped flow enclosing an angle B between 30° and 90°.

The extruder is arranged to deliver melt to the die 111 so that thecross-section of the melt 112 is equal to that of the die orifice. Thegas stream is desirably at least supersonic, possibly up to Mach 2 forbest atomisation. The particle size of the product powder is inter aliagoverned by the cross-section of the melt 112.

In a typical arrangement an extruder was used to melt PE-based polymerto a temperature of 150° C. Eight gas jet discharge orifices werearranged to define an apex angle of 45°. The nozzles were supplied withcompressed air at 0.4 Mpa. Compressed air was heated to a temperature of150° C. by a gas heater. In nozzles with cavities, there were six, eachof 1 mm diameter uniformly arranged in each gas channel, formed at anangle of 15° to the direction of the channel.

FIG. 9 shows the particle distribution of atomised polymer powderproduced by such an arrangement. The product powder is found to comprisespherical or nearly spherical particles of defined size distributiondepending on the dimension of the die orifice and the viscosity of themelt. The process can be carried out under conditions such as to avoidrisk of contamination of the product.

1. Apparatus for atomising liquid media comprising an ultrasonic gas atomisation nozzle having a gas flow path from a plenum chamber to a nozzle exit, the flow path being straight and being provided with a plurality of resonance cavities, and wherein the resonance cavities are spaced from both the plenum chamber and the nozzle exit.
 2. Apparatus according to claim 1, in which the resonance cavities are spaced apart along the gas flow path.
 3. Apparatus according to claim 1, in which the resonance cavities are inclined to the gas flow path such that the resonance cavities are convergent with the gas flow path in the direction of gas flow.
 4. Apparatus according to claim 1, in which the resonance cavities are adapted to impose an ultrasonic frequency on the gas flow.
 5. Apparatus according to claim 4, in which the frequency is in the range 20-60 KHz.
 6. Apparatus according to claim 1, in which the gas flow path comprises an annular convergent nozzle and the resonance cavities comprise cylindrical cavities formed in both interior walls of the annular nozzle.
 7. Apparatus according to claim 6, in which the diameters of the cavities are between 1/12 and ⅛ of the mean nozzle diameter.
 8. Apparatus according to claim 7, having between ten and sixty cavities.
 9. Apparatus according to claim 1, in which the gas flow path comprises a multi-jet arrangement.
 10. Apparatus according to claim 9, in which the diameters of the cavities are between 1/10 and ⅓ of the diameter of the jets.
 11. Apparatus according to claim 10, having between two and eight cavities in each jet.
 12. Apparatus according to claim 6, in which the cavities are oriented at between 10° and 60° to the flow direction through the nozzles. 